The present invention relates to a projector using laser light, and more particularly to a ring-shaped laser spot capable of forming a projected image having a ring-shaped projected image formed from a diffraction fringe, by interposing a diaphragm between a collimator lens and a target.
In recent years, a projector using laser light has been used as a guide for performing tunnel construction, installation work of water supply pipes, sewage pipes, etc., ink marking work at a building construction site, etc. In general, the laser light used in the projector is frequently aimed to decide the center by projection on an object such as a target, so that a projected image formed on the object is circular in many cases. This is due to the fact that if the projected image is elongated in a horizontal, vertical, or oblique direction to be made elliptical or rectangular, it is difficult to decide the center.
Further, the projector using laser light is less frequently used with a fixed distance to the object, but frequently used with a variable distance ranging from a short distance to hundreds of meters. Therefore, a laser spot to be projected is required to have the same size regardless of the distance to the object. To meet this requirement, the projector using laser light employs a collimated light produced by enlarging and projecting the laser light with an optical system.
By enlarging and projecting the laser light with the optical system, a projected image having substantially the same sectional shape as that of a beam of laser light emitted from a light source can be obtained. Accordingly, in a projector using a laser tube as the light source, the projected image by the laser light becomes substantially circular. On the other hand, in a projector using a semiconductor laser as the light source, the projected image by the laser light becomes substantially rectangular. The reason why the projected image obtained by the projector using the semiconductor laser as the light source is rectangular is that a laser emitting surface of the semiconductor laser is rectangular.
To make circular a beam of laser light emitted from the semiconductor laser, it is known to use an anamorphic optical system as one method and use a diaphragm as another method.
According to the method using the anamorphic optical system to make the emergent beam circular, a beam spot with a specified aspect ratio can be obtained by the anamorphic optical system, so that the emergent beam can be made substantially circular by setting the longer dimension substantially identical with the shorter dimension.
On the other hand, the method using the diaphragm to make the emergent beam circular may be realized by the configuration as shown in FIG. 8, which is composed of a semiconductor laser 1100, a focusing lens 2000, a diaphragm 3000, and a collimator lens 4000. A beam of laser light emitted from the semiconductor laser 1100 is first focused by the focusing lens 2000, and is then shaped into a circular shape by the diaphragm 3000. The diaphragm 3000 usually has an optimum opening diameter of about 2 to 3 .mu.m. An unnecessary elliptical portion of the laser beam is cut off by the diaphragm 3000, thereby making the projected image substantially circular.
It is also considered to combine the methods using the anamorphic optical system with the diaphragm, so as to make circular the emergent beam from the semiconductor laser.
However, in the method using the anamorphic optical system to make the emergent beam circular, a sufficiently large optical system must be adopted to obtain a circular beam spot by setting the longer dimension substantially identical with the shorter dimension, because the anamorphic optical system originally functions to obtain a beam spot with a specified aspect ratio. Further, the combination of the anamorphic optical system and the diaphragm brings an increase in number of parts and an increase in cost.
In the method using the diaphragm to make the emergent beam circular, it is difficult to position between the diaphragm and the semiconductor laser, and the semiconductor laser is susceptible to temperature to cause the movement of a light emitting portion. Accordingly, the opening diameter of the diaphragm in practical use must be set to about 100 .mu.m. That is, the opening diameter of the diaphragm is increased to cause a problem such that imperfect ring-shaped diffraction light is mixed in to blur the projected image, resulting in the difficulty of decision of the center.
Further, in the case of a merely uniform circular projected image, the center cannot be specified. Accordingly, the center must be decided by eye measurement in the circular projected image.
Accordingly, it is strongly desired to provide a laser spot which can easily specify the center of a projected image with a simple configuration and at a low cost.